cell wall or cell membrane depending on what type of cell
dna
bravo
looking at the cell, a city wall, as oposed to a city boundry.
The Rio Grande
A Convergence Boundary.
boundry of the cell that controlls what enters and exits the cells
The Colorado River forms the boundary between Arizona and California.
Nuclear membrane
Andes
Minnesota
Limpopa
The Colorado River forms part of the southwest boundary of the United States.